NCERT Class XII Chapter
Ray Optics and Optical Instruments
Questions With Solutions
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Question : 36
Total: 38
A Cassegrain telescope uses two mirrors as shown in figure. Such a telescope is built with the mirrors 20 mm apart. If the radius of curvature of the large mirror is 220 mm and the small mirror is 140 mm, where will the final image of an object at infinity be?
Solution:
Image formed by concave mirror acts as a virtual object for convex mirror.
Here parallel rays coming from infinity will focus at 110 mm an axis away from concave mirror.
Distance of virtual object for convex mirror = 110 – 20 = 90 mm For convex mirror
u = – 90 mm, f = – 70 mm ⇒
+
⇒ v = – 315 mm
Hence image is formed at 315 mm from convex mirror
Here parallel rays coming from infinity will focus at 110 mm an axis away from concave mirror.
Distance of virtual object for convex mirror = 110 – 20 = 90 mm For convex mirror
u = – 90 mm, f = – 70 mm ⇒
⇒ v = – 315 mm
Hence image is formed at 315 mm from convex mirror
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